1. Field of the Invention
The present invention relates generally to three-mirror telescopes and more specifically to a highly manufacturable three-mirror optical telescope in which the primary and tertiary mirrors share a common vertex and all mirrors share a common axis. The primary-tertiary mirror pair is fabricated on a common substrate with single point diamond turning, eliminating the need for aligning these two mirrors, resulting in great simplification of the fabrication and alignment processes. The optical system of the invention is particularly suitable for use as a three-mirror collimator or telescope in spacecraft applications.
2. Prior Art
Three-mirror optical systems useable as collimators and telescopes, for example in spacecraft applications, are well known in the art. One example of such a three-mirror optical system designed primarily as a telescope, is disclosed in U.S. Pat. No. 4,240,707 to Wetherell. This patent discloses radiation from an off-axis primary mirror reflected back to an on-axis convex secondary mirror, which, in turn, reflects the radiation to an off-axis tertiary mirror. The radiation is then reflected from the tertiary mirror to a focal plane, which is also off axis. Another example of a three-mirror system is shown in U.S. Pat. No. 4,265,510 to Cook, wherein a primary mirror, a secondary mirror and a tertiary mirror form an anastigmatic relayed image-forming optical system in which the image field is off the optical axis and the entrance pupil coincides with the primary mirror and an intermediate image is formed between the secondary and tertiary mirrors. Still another relevant prior art patent, namely U.S. Pat. No. 4,737,021 to Korsch, discloses a three-mirror collimator or telescope having a focal plane positioned on a first side of the optical axis and which directs a diverging beam of radiation upon a concave tertiary mirror which is also positioned on the first side of the optical axis and which reflects the received radiation to an on-axis convex secondary mirror in a converging pattern. A secondary mirror reflects the received radiation in a first converging, then diverging pattern upon a concave primary mirror which is positioned on the second side of the optical axis. The primary mirror reflects the received radiation as a collimated beam to the real entrance pupil which is located either on or near the optical axis, but which need not be precisely centered on the optical axis. The primary mirror reflects the radiation in a first converging, then diverging pattern upon the on axis secondary mirror which in turn reflects the radiation upon the off axis tertiary mirror. The tertiary mirror then reflects and focuses the received radiation upon the focal plane.
One of the principal disadvantages of all of the known three-mirror collimator or telescope optical systems of the prior art is that they are all configured in such a manner that requires each of the three mirrors to be separately fabricated and then assembled and aligned thereafter as a precise optical system. The alignment of a three-mirror system in which all three of the mirror elements has been separately fabricated is a labor-intensive task which is more conducive to error because of the need for precise placement of the three mirrors relative to one another in the system. In most high performance three-mirror systems, the stringent misalignment tolerance between mirrors, particularly the primary-tertiary pair, renders it impractical to fabricate. Consequently, a method for fabricating such a three-mirror system which reduces the complexity of the alignment and placement task, would be a highly desirable feature. Such a fabrication method is disclosed herein and constitutes a principal feature of the present invention.
Other prior art patent disclosures deemed relevant to the present invention to varying degrees comprise the following:
______________________________________ 2,729,143 White 3,801,180 MaGuire et al 4,239,342 Aurin et al 4,293,186 Offner 4,331,390 Shafer 4,469,414 Shafer 4,497,540 Breckinridge et al 4,733,955 Cook 4,737,021 Korsch 4,812,028 Matsumoto 5,009,494 Iossi et al ______________________________________
U.S. Pat. No. 4,737,021 to Korsch is relevant to the present invention in that it is directed to a three-mirror telescope arranged with a common optical axis. Referring to FIG. 2, there is shown a three-mirror telescope having an entrance pupil disposed on the optical axis, through which a collimated beam is received by the primary mirror for reflection to the on-axis secondary mirror, which in turn reflects the radiation upon the off-axis tertiary mirror for reflection to the focal plane.
U.S. Pat. No. 4,812,028 to Matsumoto is directed to a reflection type projection optical system for projecting micropattern images, such as semiconductor device patterns. Referring to FIG. 1, the reflecting surfaces M1 and M3 are formed on a common substrate with all of the mirrors in the system having a common optical axis. Although the two mirrors M1 and M3 have a common radius of curvature, the two mirrors have a common vertex.
U.S. Pat. No. 4,293,186 to Offner is directed to an off-axis optical system. Although directed to a projection system, the optical system comprises two spherical mirrors, a convex mirror and a concave mirror, wherein the system is arranged to provide three reflections. The mirrors are arranged with their centers of curvature along the system axis with the concave mirror having a portion above the axis providing one set of reflections and another portion below the axis providing another set of reflections.
U.S. Pat. No. 4,733,955 to Cook is directed to a reflective optical triplet for a telescope. Referring to FIGS. 3 and 4, there is shown, the plan and elevation views for the optical system wherein the radiation beam enters through the real pupil for reflection from the primary mirror upon the secondary mirror and subsequently the tertiary mirror, for impinging upon a sensor. The optical elements are arranged to have a common optical axis. The primary and tertiary mirrors are not disclosed as being formed on a common substrate.
U.S. Pat. No. 4,497,540 to Breckinridge et al is directed to an optical system for viewing a remote surface. FIG. 7 illustrates an embodiment wherein the optical elements are arranged so as to have a common optical axis and a spherical mirror is defined by two reflecting surface portions. However, the surface portions each have the same radius of curvature.